Mass. Gen. Laws ch. 231 § 85U
No person who is a lawful occupant of a dwelling shall be liable in an action for damages for death or injuries to an unlawful occupant of said dwelling resulting from the acts of said lawful occupant; provided, however, that said lawful occupant was in the dwelling at the time of the occurrence and that he acted in the reasonable belief that the person unlawfully in said lawful dwelling was about to inflict great bodily injury or death upon said occupant or upon another person lawfully in said dwelling, and that said lawful occupant used reasonable means to defend himself or such other person lawfully in said dwelling. There shall not be a duty on said occupant to retreat from such person unlawfully in said dwelling.
Mass. Gen. Laws ch. 231, § 85U